Folding tools such as knives generally include a handle comprised of opposed handle halves that are held apart to define a blade-receiving space. A blade is pivotally attached to the handle with a pivot shaft extending between the opposed handle halves and through a bore in the blade thereby defining a connection between blade and handle. The blade may therefore be pivoted between the opposing handle halves from a closed position, in which the blade is stowed safely in the blade receiving space of the handle, and an open position in which the blade extends away from the handle into a position for use.
There are different kinds of structures used for pivot shafts, the oldest being a simple cylindrical post with ends press fit into bores in the opposing handle halves. In some variations, the ends of the pivot shaft extend completely through each handle half and the ends are peened to form heads that prevent the pivot shaft from moving back through the bore, thereby securing the opposite handle halves generally into a predetermined position.
The position of the handle halves with respect to each other is important because it determines how freely the blade will move between the opposing handle halves and can result in conditions ranging from what is colloquially known as “loose” wherein the blade can perceptibly wobble between the opposing handle halves, and “tight” wherein blade movement is impaired by excess friction between the blade the handle halves.
An inherent problem experienced in pivoting knives (and other folding tools) is that any pre-determined tolerance between handle halves is transient when the blade (or tool) is submitted to hard use. Pivoting blades loosen rapidly when the pivot receives axially directed loading from torsion and eccentric tensile loading exerted upon the blade. Furthermore, longitudinal shear loads are transmitted to the handle halves through the pivot ends. This induces deflection within the assembly that greatly contributes to blade loosening.
Traditionally, the solution for blade loosening is occasional readjustment through pressing or re-peening the solid pivot post. Repeated often, however, deformation of the pivot ends and surrounding material caused by this adjustment method results in fatigue and eventually material failure at the pivot ends and the corresponding bores.
Adjustable pivots have been developed to provide a more elegant and repeatable solution to the problem of blade loosening.
While there are different variations known to the art, adjustable pivots for folding knives and tools basically function by squeezing the handle halves together against the blade through compressive axial loading provided by interconnecting adjustable members. Most commonly, this pivot arrangement includes a pivot shaft with internally threaded axial bore opening at one end and a concentric head of greater diameter terminating the other end. Also included is an adjustment screw configured to threadedly interact with the aforementioned internally threaded axial bore. In use, the pivot shaft is fitted through a bore in each opposing handle half with blade pivotally fitted there between. The concentric head of the pivot shaft prevents the pivot shaft from passing completely thorough the bore in one handle half. The assembly is completed when the adjustment screw is inserted through the available bore in the other handle half and into the internally threaded axial bore of the pivot shaft.
Adjustment is facilitated because the pivot shaft is configured to be too short to extend completely through both handle halves while the blade is in place there between. As a result, the pivot shaft extends completely through one handle half and the blade, but only partially through the opposite handle half. Therefore, as the pivot screw is tightened, the pivot shaft is free to move within the partially occupied bore, drawing the handle halves together thereby inducing variable friction upon the blade.
This prior art pivot is well documented in the art and can be found often associated with the disclosures of other inventions. For example, in disclosing a Folding Knife with Safety Device, U.S. Pat. No. 7,165,329 to Kao clearly presents an adjustable pivot typical of the prior art on drawing sheets 4 and 5 of that patent. Similarly, U.S. Pat. No. 7,146,736 to Collins includes drawing sheet 1 depicting this prior art adjustable pivot while teaching a Folding Knife With Cantilevered Spring. U.S. Pat. No. 7,325,312 to Janich for a Folding Knife with Pivoting Blade and Guard shows this prior art adjustable pivot on drawing sheets 2 and 4.
Indeed, the concept of pinching a pivoting blade (or tool) between handle halves, the blade itself serving as spacer between the handle halves, is ubiquitous. The same operational concept underlies many otherwise novel folding knife pivots of record. For example, U.S. Pat. No. 6,101,723 to Ford teaches a Folding Knife With Eccentric Pivot Pin and U.S. Pat. No. 7,905,023 to Westerfield teaches an Adjustable Diameter Pivot Shaft For Hand Tool. Despite their elaborate solutions to different pivot-related problems, all function the same in that they pinch the blade between handle halves and use the pivoting blade as spacer there between.
The primary deficiency with all adjustable folding knife pivots heretofore is that they cannot provide substantial structural support between the two handle halves at the pivot point, resulting in a tendency for the blade to loosen within the assembly when subjected to stress. That is because, unlike a static spacer or standoff that provides normal support against a fully tightened fastener, the adjustable pivots of prior art cannot provide a rigid point of connection between handle halves. Instead, the blade itself governs the distance between the handle halves at this critical location and it must be loose enough to allow the blade to move freely. A loose screw provides both the means of blade adjustment and the primary means for resisting axially directed tensile loads induced between handle halves. The result is insufficient rigidity at the pivot point that allows deflection from shear and torsion loads to rapidly degrade blade adjustment.